Mechanical alterations in shale formations due to exposure to water-based fracturing fluids and supercritical carbon dioxide (ScCO2) significantly affect the performance of shale gas exploration and CO2 geo-sequestration. In this study, a hydrothermal (HT) reaction system was set up to treat Longmaxi shale samples of varying mineralogies (carbonate-, clay-, and quartz-rich) with different fluids, i.e. deionized (DI) water, 2% potassium chloride (KCl) solution, and ScCO2 under HT conditions expected in shale formation. Statistical micro-indentation was conducted to characterize the mechanical property alterations caused by the shale-fluid interactions. An in situ morphological and mineralogical identification technique that combines scanning electron microscopy (SEM) and backscattered electron (BSE) imaging with energy-dispersive X-ray spectroscopy (EDS) was used to analyze the microstructural and mineralogical changes of the treated shale samples. Results show no apparent changes in the Young's modulus, E, and hardness, H, after treatment with DI water under room temperature (20 degrees C) and atmospheric pressure for 7 d. In contrast, E and H were decreased by 31.2% and 37.5% at elevated temperature (80 degrees C) and pressure (8 MPa), respectively. The addition of 2% KCl into DI water mitigated degradation of the mechanical properties. Quartz-rich shale specimens are the least sensitive to the water-based fracturing fluids, followed by the clay-rich and carbonate-rich shale formations. Based on in situ morphological and mineralogical identification, the primary factors for the mechanical degradation induced by water-based fluids include carbonate dissolution, clay swelling, and pyrite oxidation. Slight increases in the measured E and H and compression of porous clay aggregates were observed after treatment with ScCO2. The major factor contributing to the mechanical changes resulting from the exposure to scCO2 appears to be the competition between swelling caused by adsorption and compression of shale matrix. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

To investigate the expansion deformation of predisintegrated shale as subgrade soil and develop a calculation model to describe its expansion characteristic, the nonloaded and loaded expansion tests as well as expansion force tests were conducted on the typical predisintegrated shale under varying conditions. The results show that the expansion rate of compacted predisintegrated shale is greatly affected by initial water content, dry density, and overburden load, and that its expansion characteristics can be divided into three stages. After compaction, the expansion force of predisintegrated shale has a strong correlation with the initial dry density and initial water content, and 1.45 gcm(-3) can be used as threshold density for the rate of expansion force growth. Moreover, a comprehensive calculation model for the expansion rate and expansion force considering the coupling effects of various factors was proposed. This study proposes an alternative subgrade material using shale, which can efficiently save land resources and reduce engineering costs.

The loaded rock experiences multiple stages of deformation. It starts with the formation of microcracks at low stresses (crack initiation, CI) and then transitions into unstable crack propagation (crack damage, CD) near the ultimate strength. In this study, both the acoustic emission method (AEM) and the ultrasonic testing method (UTM) were used to examine the characteristics of AE parameters (b-value, peak frequency, frequency-band energy ratio, and fractal dimension) and ultrasonic (ULT) properties (velocity, amplitude, energy attenuation, and scattering attenuation) of bedded shale at CI, CD, and ultimate strength. The comparison involved analyzing the strain-based method (SBM), AEM, and UTM to determine the thresholds for damage stress. A fuzzy comprehensive evaluation model (FCEM) was created to describe the damage thresholds and hazard assessment. The results indicate that the optimal AE and ULT parameters for identifying CI and CD stress are ringing count, ultrasonic amplitude, energy attenuation, and scattering attenuation of the S-wave. Besides, damage thresholds were detected earlier by AE monitoring, ranging from 3 MPa to 10 MPa. CI and CD identified by UTM occurred later than SBM and AEM, and were in the range of 12 MPa. The b-value, peak frequency, energy ratio in the low-frequency band (0-62.5 kHz), correlation dimension, and sandbox dimension showed low values at the peak stress, while the energy ratio in a moderate-frequency band (187.5-281.25 kHz) and amplitude showed high values. The successful application of FCEM to laboratory testing of shales has demonstrated its ability to quantitatively identify AE/ULT precursors of seismic hazards associated with rock failure. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The study of the effects of supercritical CO2 (ScCO2) under high temperature and high pressure on the mechanical properties and fracturing potential of shale holds significant implications for advancing our understanding of enhanced shale gas extraction and reservoir exploration and development. This study examines the influence of three fluids, i.e. ScCO2, deionized water (DW), and ScCO2+DW, on the mechanical properties and fracturability of shale at immersion pressures of 15 MPa and 45 MPa, with a constant temperature of 100 C. The key findings are as follows: (1) Uniaxial compressive strength (UCS) of shale decreased by 10.72%, 11.95%, and 23.67% at 15 MPa, and by 42.40%, 46.84%, and 51.65% at 45 MPa after immersion in ScCO2, DW, and ScCO2+DW, respectively, with the most pronounced effect observed in ScCO2+DW; (2) Microstructural analysis revealed that while ScCO2 and DW do not significantly alter the microstructure, immersion in ScCO2+DW results in a more complex surface morphology; (3) Acoustic emission (AE) analysis indicates a reduction in stress for crack damage, with a decreased fractal dimension of AE signals in different fluids. AE energy is primarily generated during the unstable crack propagation stage; (4) A quantitative method employing a multi-factor approach combined with the brittleness index (BI) effectively characterizes shale fracturability. Evaluation results show that ScCO2+DW has a more significant effect on shale fracturability, with fracturability indices of 0.833% and 1.180% following soaking at 15 MPa and 45 MPa, respectively. Higher immersion pressure correlates positively with increased shale fracturability. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

The stimulation of shale reservoirs frequently involves significant shear failure, which is crucial for creating fracture networks and enhancing permeability to boost production. As the depth of extraction increases, the impact of elevated temperatures on the anisotropic shear strength and failure mechanisms of shale becomes pronounced, yet there is a notable lack of relevant research. This study conducts, for the first time, direct shear experiment on shales at four different temperatures and seven bedding angles. By employing acoustic emission (AE) and digital image correlation (DIC) techniques, the evolution of damage and the mechanism of crack propagation under anisotropic direct shearing at varying temperatures is revealed. The results indicate that both shear displacement and strength of shale increase with temperature across different bedding angles. Additionally, shale demonstrates distinct brittle failure characteristics under various conditions during direct shearing tests. The types of anisotropic shear failure observed under the influence of temperature include central shearing fracture, central shearing with secondary fracture, and deflected slip along the bedding. Moreover, the temperature effect enhances shear-induced crack propagation along bedding planes. Shear failure in shale predominantly occurs during higher loading stages, which coincide with a substantial amount of AE signals. Finally, the introduction of the anisotropy index and temperature sensitivity coefficient further elucidates the interaction mechanism between thermal effects and anisotropy. This study offers a novel methodology to explore the anisotropic shear failure behavior of shale under elevated temperatures, and also provides crucial theoretical and experimental insights into shear failure behavior relevant to practical shale reservoir stimulation. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Shale formations have recently gained plenty of attention owing to their large amounts of reserves. Horizontal drilling and hydraulic fracturing are the proposed approaches for the development of shale formations. The extended information of the mechanical properties of shale formation is crucial for designing a successful hydraulic fracturing operation. On the other hand, the mechanical properties of such organic-rich formations are greatly affected by the mechanical characteristics of the present kerogen (organic matter), which dramatically changes during the maturation process. In this study, a Qingshankou shale sample containing kerogen type I is mechanically investigated at different maturity levels using the grid nanoindentation approach. To this end, the original immature sample is artificially matured during hydrous (HP) and anhydrous (AHP) pyrolysis. More than 930 nanoindentation tests were performed on grids of 9 x 8 on the surface of 13 samples with different maturities. The test results showed that the presence of water during pyrolysis can significantly affect the shale sample's mechanical characteristics. In higher temperatures and higher levels of maturity, the role of water becomes more pronounced. During hydrous pyrolysis, kerogen produces larger amounts of oil and bitumen, which become progressively porous. While the original sample showed a Young's modulus value of more than 48 GPa, and it fluctuated between approximately 19 and 32 GPa during the HP scenario and between 17 and 34 GPa during the AHP process. In terms of hardness, the original sample exhibited an initial value of about 1.1 GPa and more mature samples reflected hardness values in the range of approximately 0.3 and 0.97 GPa in both scenarios. According to the trends of mechanical properties during maturation, mechanical properties decreased at the initial stage of maturation and remained relatively constant during the oil window. Then, another decline was detected at the wet-gas window's closure. In the dry-gas window, HP and AHP scenarios exhibited different behaviors mainly due to the chemical structure of the kerogen residue. 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Controllable shock wave fracturing is an innovative engineering technique used for shale reservoir fracturing and reformation. Understanding the anisotropic fracture mechanism of shale under impact loading is vital for optimizing shock wave fracturing equipment and enhancing shale oil production. In this study, using the well-known notched semi-circular bend (NSCB) sample and the novel double-edge notched flattened Brazilian disc (DNFBD) sample combined with a split Hopkinson pressure bar (SHPB), various dynamic anisotropic fracture properties of Lushan shale, including failure characteristics, fracture toughness, energy dissipation and crack propagation velocity, are comprehensively compared and discussed under mode I and mode II fracture scenarios. First, using a newly modified fracture criterion considering the strength anisotropy of shale, the DNFBD specimen is predicted to be a robust method for true mode II fracture of anisotropic shale rocks. Our experimental results show that the dynamic mode II fracture of shale induces a rougher and more complex fracture morphology and performs a higher fracture toughness or fracture energy compared to dynamic mode I fracture. The minimal fracture toughness or fracture energy occurs in the Short-transverse orientation, while the maximal ones occur in the Divider orientation. In addition, it is interesting to find that the mode II fracture toughness anisotropy index decreases more slowly than that in the mode I fracture scenario. These results provide significant insights for understanding the different dynamic fracture mechanisms of anisotropic shale rocks under impact loading and have some beneficial implications for the controllable shock wave fracturing technique. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Layered rock formations are frequently encountered during the excavation of underground structures. The stability of such structures is influenced not only by the stress concentration caused by the cavities in the strata but also by the anisotropy of the layered rock mass. The interaction between them can lead to critical structural failure, such as rupture, collapse, or significant deformation within the adjacent rock mass, thereby jeopardizing operational safety. However, the coupling law and mechanism between the stress concentration resulting from the cavities and the anisotropy of a layered rock mass remain unclear. In this study, a uniaxial compression test was performed on shale specimens containing a circular hole to investigate the effects of layer inclination and circular holes on the mechanical properties, elastic energy storage, and failure behaviors of these specimens. The failure mechanism of the rock surrounding the hole was analyzed on the basis of the single plane of weakness theory and the Kirsch solution. The test results indicated pronounced anisotropy in the compressive strength, elastic modulus, and elastic strain energy of the specimens, with distinct V, M and U-shaped patterns correlated with varying layer inclination angles. In addition, the combined effect of stress concentration and layer inclination resulted in different failure types, which were classified into four groups according to their failure behavior. Theoretical analysis revealed that failure around circular holes in layered rock is affected by a range of variables, such as layer inclination, layer strength, lateral pressure coefficient, azimuth, and loading stress. (c) 2025 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Biocementation is an emerging field within geotechnical engineering that focuses on harnessing microbiological activity to enhance the mechanical properties and behavior of rocks. It often relies on microbial-induced carbonate precipitation (MICP) or enzyme-induced carbonate precipitation (EICP) which utilizes biomineralization by promoting the generation of calcium carbonate (CaCO3) within the pores of geomaterials (rock and soil). However, there is still a lack of knowledge about the effect of porosity and bedding on biocementation in rocks from a mechanistic view. This experimental study investigated the impact of porosity and bedding orientations on the mechanical response of rocks due to biocementations, using two distinct biocementation strategies (MICP and EICP) and characteristically low porosity but interbedded rocks (shale) and more porous but non-bedded (dolostone) rocks. We first conducted biocementation treatments (MICP and EICP) of rock samples over a distinct period and temperature. Subsequently, the rock strength (uniaxial compressive strength, UCS) was measured. Finally, we analyzed the preand post-treatment changes in the rock samples to better understand the effect of MICP and EICP biocementations on the mechanical response of the rock samples. The results indicate that biocementations in dolostones can improve the rock mechanical integrity (EICP: +58% UCS; MICP: +25% UCS). In shales, biocementations can either slightly improve (EICP: +1% UCS) or weaken the rock mechanical integrity (MICP: -39% UCS). Further, results suggest that the major controlling mechanisms of biogeomechanical alterations due to MICP and EICP in rocks can be attributed to the inherent porosity, biocementation type, and bedding orientations, and in few cases the mechanisms can be swelling, osmotic suction, or pore pressurization. The findings in this study provide novel insights into the mechanical responses of rocks due to MICP and EICP biocementations.

Understanding the mechanical behaviour of natural soils as mixed with cement for stabilization is crucial for civil engineering developments. The response of cement-soil admixtures when subjected to cyclic loads is a largely unexplored topic, despite the importance of understanding fatigue in these ubiquitous construction materials. We present cyclic loading experiments on Portland cement mixed with fragmented shale fragments using triaxial testing, monitored with synchrotron-based mu CT. Through digital volume correlation (DVC), the temporal evolution of the displacement, volumetric, and von Mises equivalent strain fields were obtained. We observed in detail the fatigue damage evolution during cyclic loading and found that following high-strain deformation of the much softer shale fragments, the ultimate failure of the samples occurred in the adjacent cement matrix. The failure mechanism under periodic stress and its relevance for accelerated laboratory testing of slow degradation long-term processes are of key importance to technical infrastructure, including subsea CO2 storage.